1. Field of the Invention
The invention relates in general to a liquid crystal display (LCD) driving apparatus and the method thereof, and in particular to an LCD driving apparatus and the method thereof having improved displaying quality.
2. Description of the Related Art
Liquid crystal displays (LCDs) have been widely used for their characteristics of lightness and thinness. However, the LCDs have slow speed of responding, as compared with the traditional cathode ray tube (CRT) monitor. The LCD tends to have image residue as the dynamic images are displayed, while the CRT monitor does not.
The way that the CRT monitor displays the frames is called an impulse type. Each pixel only emits light at an instant during each frame period. Referring to FIG. 1, it shows the relation of lightness I for one pixel vs. time t of the CRT monitor. The pixel values D of this pixel at frame period T1, T2, and T3 are supposed to be respectively 34, 100, and 30. The illumination intensities of pluses 11 are controlled according to the pixel values D. The lightness of the present frame period will not affect that of the next frame period as a consequence of the impulse type, and thus the image residue is not existed and the response time is short.
The way that the LCD displays the frames is called a hold type. Each pixel value D emits constant light in one frame period. Referring to FIG. 2A, it shows the relation of time t and driving voltage Vd applied to the pixel according to the display of LCD. The pixel values D of the pixel at frame periods T1, T2, and T3 are supposed to be respectively 34, 100, and 30. The driving voltages Vd at frame period T1, T2, and T3 are respectively determined according to those pixel values D.
Referring to FIG. 2B, it shows the diagram of the lightness L of the pixel vs. time t. The lightness line 21 is the ideal lightness of the pixel according to the driving voltage Vd of FIG. 2A. In reality, the response speed of the liquid crystal molecule is slower than that of the electric field, and thus a response time is required for the pixel to reach the proposed lightness. The lightness line 22 is the actual lightness of the pixel according to the driving voltage Vd of FIG. 2A. The quality of image is lowered with the image residue caused by the slow response.
The above problem can be improved, for example, by over-driving method. If the pixel value of the present frame period to be displayed is larger than that of the previous one, the driving voltage larger than that to be displayed is applied to the pixel. If the pixel value of the present frame period to be displayed is smaller than that of the previous one, the driving voltage smaller than that to be displayed is applied to the pixel.
However, the display quality of LCD is still not as satisfying as the CRT even if the liquid crystal molecule responds to the applied driving voltage in real time due to the hold type. For example, the image at the beginning of the frame period T3 will overlaps with the image of the frame period T2 by human's eye, when the responding is supposed to be real time according to the lightness lines 21 of FIG. 2B. Therefore, not only the low speed of responding, but the hold type also decreases the displaying quality of the LCD.